This application is based on Japanese Application No. 10-213695, the content of which is hereby incorporated by reference.
The invention relates to a reflection-type liquid crystal optical modulation element and a projection display device for projecting a color image on a screen using the reflection-type liquid crystal optical modulation element.
Liquid crystal optical modulation elements which may be used as optical modulators, include a transmission-type and reflection-type, both of which are subject to demand for panel compactness and increased number of display pixels. In the case of identical size panels, the aperture ratio can generally be reduced as the number of pixels increases; but since the pixel drive wiring and the like can be provided on the back side of reflective electrodes in liquid crystal optical modulators of the reflection-type elements, the aperture ration can be increased as the number of pixels is increased. Thus, compactness can be achieved at the same aperture ratio and the same number of pixels. As a result, the reflection-type liquid crystal optical modulators are advantageous for high definition television (HDTV) formats such as digital television, and high quality displays can be attained if the reflective type optical modulators are used.
Devices which display a color image using liquid crystal optical modulators are either a three-panel type using three liquid crystal optical modulators corresponding to red (R), green (G), and blue (B) (xe2x80x9cRGBxe2x80x9d), respectively, or a single-panel type using a single liquid crystal optical modulator. The single-panel type devices can be made more compact and are extremely advantageous from a cost perspective since they use fewer components. However, in order to use a color absorption filter having an RGB pattern for colorization in a liquid crystal optical modulator, approximately two-thirds of the light is wasted. For this reason, a liquid crystal optical modulator has been processed which used a diffraction element (e.g., hologram) as a color filter to eliminate the aforesaid disadvantage.
FIG. 13 shows an example of a conventional transmission type liquid crystal optical modulator using a color filter comprising a diffraction element. White illumination light L1 is condensed by an array of micro lenses, and diffracted into a spectrum in the RGB wavelength bands by a unidimensional diffraction element D0. The respective diffracted light fluxes pass through the pixels corresponding to a liquid crystal layer 2, and are used to display a color image. In this way, the majority of the liquid crystal optical modulator using a diffraction element as a color filter is the transmission type. This results in the disadvantages described below when a color filter comprising a diffraction element is used in a reflective type liquid crystal optical modulator.
As an example, in a display device which projects a color image using reflective type liquid crystal optical modulators, unlike transmissive type liquid crystal optical modulators, the illumination light and projections are typically separated between the liquid crystal surface and the projection surface via a polarizing prism or the like. In the case of a three-panel liquid crystal optical modulation element, since the illumination light enters the liquid crystal optical modulator after color separation, the principal ray of the light flux entering the respective elements passes through the condensing micro lenses, enters the pixel of the liquid crystal layer perpendicularly, and subsequently subjected to regular reflection by the reflective electrode and returns to the same micro lens.
Conversely, when a single-panel liquid crystal optical modulator uses a diffraction element as a color filter, the principal rays of at least two of the light fluxes among the RGB light fluxes diffracted into spectral light via the diffraction/dispersion action of the diffraction element enter at a predetermined angle relative to the reflective electrode surface. For example, when illumination light L1 passes through condensing micro lenses 1 and spectral diffraction element D0, illumination light L1 passes through first substrate P1 and liquid crystal layer 2, and among the light fluxes which condense and impinge reflective electrode 3 and second substrate P2, the principal rays of the R and G light flux impinge reflective surface of reflective electrode 3 at predetermined angles. In this way, part of the R and G light components are subject to regular reflection by the reflective surface as by a normal mirror, and is reflected in the direction of the other pixels. Since this light is used as stray light LS together with projection light L2 to display a color image, cross-talk is generated for each picture element, thus greatly reducing the display quality of the projection image.
In U.S. Pat. Nos. 5,737,113 and 5,608,552, liquid crystal optical modulators are proposed to eliminate these disadvantages. This construction, however, generates new disadvantages which must be resolved. For example, in the former liquid crystal optical modulator, a complex construction is necessary to drive the image signals of the liquid crystal via a method which differs from conventional methods. In the latter liquid crystal optical modulator, a complex hologram having a 4-layer structure is required due to the necessity of having the illumination light enter at an inclination. Furthermore, the hologram must be thickened to achieve adequate separation of the P and S polarized light components, thereby increasing the dependency of the diffraction efficiency of the hologram on the wavelength and the angle.
The invention provides an improved liquid crystal optical modulation element and projection display device. The invention further provides a reflective-type liquid crystal optical modulation element capable of displaying color at high display quality via a simple construction without using an absorption type color filter. In addition, the invention provides an inexpensive single-panel projection display device capable of displaying a bright and high quality color.
A liquid crystal optical modulation element in accordance with an embodiment of the invention includes spectral means for diffracting white illumination light into light of predetermined wavelength bands, a liquid crystal layer for modulating the intensity of the light entering a plurality of two-dimensional pixels, condensing means for condensing the light spectrally diffracted by the spectral means so as to direct the light to each pixel of the liquid crystal layer, and reflecting means for reflecting the light which pass through the liquid crystal layer in approximately the same direction as the entering light.